US President Donald Trump's social media director Dan Scavino broke the law in April in calling for Trump supporters to defeat a Republican congressman at the polls, according to a letter from the US Office of Special Counsel.
The office found that Mr Scavino had violated the Hatch Act, which prohibits executive branch employees from engaging in some forms of political activity, and it issued a warning to him, it said in the letter.
Politico on Friday was the first to report on the contents of the June 5 letter, which was delivered to an organisation that had brought a complaint.
In a tweet on April 1, Mr Scavino had urged voters to defeat Republican Representative Justin Amash. Mr Amash is a member of the conservative House Freedom Caucus, which had helped beat back an early version of a healthcare reform bill that was a top priority for the President.
"@justinamash is a big liability. #TrumpTrain, defeat him in primary," Mr Scavino tweeted.
"Mr Scavino has been advised that if in the future he engages in prohibited political activity while employed in a position covered by the Hatch Act, we will consider such activity to be a wilful and knowing violation of the law," Ana Galindo-Marrone, chief of the Office of Special Counsel's Hatch Act Unit, wrote in the letter.
The office is an agency independent from the Justice Department that investigates acts in which government employees commit "prohibited personnel practices."